Heretofore, various kinds of paper containing soft-magnetic microwires (tag) have been studied for the prevention of forgery, security for highly confidential information, and other purposes.
The related-art technology for the paper mentioned is overviewed. U.S. Pat. No. 7,301,324 describes a recording medium detecting system including a magnetic field-generating unit that generates an alternating magnetic field in a predetermined particular region, a detecting unit provided close to the particular region for detecting a change in magnetic flux, and a recording medium, that is detectable by the detecting unit when placed in the particular region. The recording medium may be a two-layered or three-layered structure containing magnetic wires. In the two-layer structure, the magnetic wires are placed on one face of a substrate previously prepared and laminating another substrate hereon. In the three-layered structure a single-layered substrate (or wet paper) containing dispersed wires is sandwiched between two substrates (or wet paper) containing no wires. According to this technique, a plurality (eighteen) of 25 mm-long magnetic wires are dispersed uniformly over a whole sheet of wet paper. The wire has a diameter of 30 μm and a coercive force of less than 70 A/m.
Further, WO14185686 and KR101341164 describe a recording medium for security, including a detection base layer containing a material for detection and having a printable base integrated with the material for detection; and a shielding layer formed on both sides of the detection base layer so as to shield the material for detection. Here, the material for detection can be a soft magnetic material having a residual magnetic flux density of 0.1 T or more, a coercive force of 1 to 50 A/m and a magnetic permeability of 500 to 100,000H/m, when the frequency is 10 kHz.